1. Field of the Invention
The present invention relates to wireless networks, more specifically to dual-mode wireless telephone communication networks that overlay digital cellular systems over existing analog-based cellular systems.
2. Background Art
Wireless telephone communication systems have evolved from the Advanced Mobile Phone Service (AMPS) technology, introduced around 1983, to more sophisticated digital-based air interface protocols. Specifically, the AMPS technology is an analog-based technology that uses frequency division multiple access techniques to divide the available radio spectrum into channel bandwidths, where each 30 kilohertz voice channel supports a single conversation.
FIG. 1 is a diagram illustrating a cellular communication system including an AMPS-based wireless telephone system 10. As shown in FIG. 1, the AMPS system 10 includes an AMPS compliant base station 12a and a plurality of transceivers 14 configured for transmitting voice channels within the 800 MHz cellular band into overlapping propagation regions 16, also referred to as cells. The arrangement of the antennas 14 at prescribed geographic locations establish a cellular-based coverage area
Digital access technologies have been developed based on Time Division Multiple Access (TDMA) or Code Division Multiple Access (CDMA) schemes. These digital access technologies are more efficient relative to analog-based access technologies such as AMPS. In TDMA, the RF carrier is divided into time slots. The digital system allocates speech to these time slots by converting and compressing speech information carried by analog voice signals into compressed digital samples using vocoding techniques, enabling the speech samples to fit into the time slots. Hence, 3 or more time slots may fit into each 30 kHz channel, resulting in at least a three-fold increase in capacity. The well-recognized TDMA-based GSM standard utilizes 8 time slots in 200 kHz bandwidth channels.
Code Division Multiple Access (CDMA) is an even more efficient access technology, where the compressed speech samples are spread over a large frequency band by coding each speech sample with a unique pseudorandom sequence. Multiple voice samples can then be sent through the same wide-frequency band. The digital wireless telephone having the correct pseudorandom sequence will reproduce the corresponding speech sample, whereas voice samples transmitted with different pseudorandom sequences will appear as background noise in the frequency band.
Hence, digital access technologies have substantial advantages over analog-based systems. Digital cellular systems, however, have not yet been deployed in as many regions as AMPS-based systems. Hence, digital cellular subscribers are offered continuous coverage using dual-mode wireless telephones capable of switching between a first mode (e.g., CDMA) and a second mode (e.g., AMPS) based on the availability of a certain technology.
As shown in FIG. 1, a dual-mode wireless communications system 20 includes an analog base station 12b of an analog wireless communications system, a digital cellular system (e.g., CDMA) 22, and a plurality of antennas 24 configured for transmitting both wireless telephone signals from the AMPS base station 12b and the CDMA system 22. Hence, the AMPS base station 12b transmits the analog cellular signals 26 into the appropriate cells 28 via the respective transceiver antennas 24, establishing an analog-based wireless telephone system in the coverage areas 28. Similarly, the CDMA system 22 establishes a digital wireless system by transmitting the CDMA-encoded signals 30 to the same antennas 24 (or separate co-located antennas), enabling the digital wireless system to overlie the prescribed coverage areas 28 of the analog-based system generated by the AMPS base station 12b. Hence, a dual-mode wireless telephone 32 is able to initiate and receive telephone calls within the cell sites 28 using the CDMA system 22, and within the cell sites 16 using the AMPS system 12a AMPS wireless telephones (not shown) can initiate and receive telephone calls in the cell sites 28 using the AMPS system 12b.
A problem encountered in the dual-mode system 20 occurs when the dual-mode wireless telephone moves across a boundary 34 between cellular systems 10 and 20. In particular, assume the dual-mode telephone 32 is in active communication with the CDMA base station 22 for a call in progress while the subscriber moves across the border 34 from the cell sites 28 of the dual-mode system 20 to the cell sites 16 of the AMPS system 10. As recognized in the art, the movement of the dual-mode wireless telephone 32 across the boundary 34 requires a hard handoff by the dual-mode telephone with the AMPS-based system 10. The hard handoff from the CDMA-based system 22 within the dual-mode system 20 to the AMPS-based system 10 is one of the most complicated and signaling-intensive procedures in wireless networks, especially since the dual-mode wireless telephone 32 must switch from digital mode to analog mode simultaneously with the hard handoff. Hence the hard handoff from the CDMA system 22 to the AMPS system 10 has a substantial risk in dropping the call.
Although there is a desirability to minimize the risk in dropped calls, current dual-mode digital telephones 32 are configured for initially searching for the preferred digital technology, such as CDMA. Hence, the dual-mode wireless telephone 32 will connect with the alternative technology (e.g., AMPS) only if the dual-mode digital telephone 32 is unable to establish a link with the preferred digital (e.g., CDMA) technology. Hence, the dual-mode telephone 32 will always try to look for the preferred CDMA technology first. Consequently, the dual-mode telephone 32 cannot be controlled to avoid establishing a CDMA-based call while the digital telephone 32 is within the dual-mode cellular system 20, even if there is a substantial probability that the dual-mode telephone 32 will move across the boundary 34 into the AMPS-based system 10, risking a dropped call.